1. Field of the Invention
The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to an alkali metal cell suitable for current pulse discharge applications with reduced or no appreciable voltage delay. Still more particularly, the present invention relates to a lithium electrochemical cell activated with an electrolyte having an additive for the purpose of reducing and/or eliminating voltage delay under current pulse discharge applications. Voltage delay is a phenomenon typically exhibited in an alkali metal/transition metal oxide cell, and particularly, a lithium/silver vanadium oxide cell, that has been depleted of 40% to 70% of its capacity and is subjected to current pulse discharge applications. According to the present invention, the preferred additive to the activating electrolyte for such a chemistry is a phosphate compound.
The voltage response of a cell which does not exhibit voltage delay during the application of a short duration pulse or pulse train has distinct features. First, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse, and second, the minimum potential of the first pulse in a series of pulses is higher than the minimum potential of the last pulse. FIG. 1 is a graph showing an illustrative discharge curve 10 as a typical or "ideal" response of a cell during the application of a series of pulses as a pulse train that does not exhibit voltage delay.
On the other hand, the voltage response of a cell which exhibits voltage delay during the application of a short duration pulse or during a pulse train can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the first pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied. FIG. 2 is a graph showing an illustrative discharge curve 12 as the voltage response of a cell that exhibits both forms of voltage delay.
The initial drop in cell potential during the application of a short duration pulse reflects the resistance of the cell, i.e., the resistance due to the cathode-electrolyte interphase and the anode-electrolyte interphase and due to polarization. In the absence of voltage delay, the resistance due to passivated films on the anode and/or cathode is negligible. However, the formation of a surface film is unavoidable for alkali metal, and in particular, lithium metal anodes, and for lithium intercalated carbon anodes, due to their relatively low potential and high reactivity towards organic electrolytes. Thus, the ideal anode surface film should be electrically insulating and ionically conducting. While most alkali metal, and in particular, lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. In the event of voltage delay, the resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation which often results in reduced discharge voltage and reduced cell capacity. In other words, the drop in potential between the background voltage and the lowest voltage under pulse discharge conditions, excluding voltage delay, is an indication of the conductivity of the cell, i.e., the conductivity of the cathode, anode, electrolyte, and surface films, while the gradual decrease in cell potential during the application of the pulse train is due to the polarization of the electrodes and electrolyte.
Thus, the existence of voltage delay is an undesirable characteristic of alkali metal/mixed metal oxide cells subjected to current pulse discharge conditions in terms of its influence on devices such as medical devices including implantable pacemakers and cardiac defibrillators. Voltage delay is undesirable because it limits the effectiveness and even the proper functioning of both the cell and the associated electrically powered device under current pulse discharge conditions.
2. Prior Art
One of the known solutions to the above problem is to saturate the electrolyte solution with carbon dioxide CO.sub.2. Cycling efficiency is improved dramatically in secondary cell systems having a lithium anode activated with CO.sub.2 saturated electrolytes (V. R. Koch and S. B. Brummer, Electrochimica Acta, 1978, 23, 55-62; U.S. Pat. No. 4,853,304 to Ebner et al.; D. Aurbach, Y. Gofer, M. Ben-Zion and P. Aped, J. Electroanal. Chem. 1992, 339, 451-471). U.S. Pat. No. 5,569,558 to Takeuchi et al. relates to the provision of a CO.sub.2 saturated electrolyte for alleviating the presence of voltage delay in primary cells having a mixed transition metal oxide cathode such as lithium/silver vanadium oxide cells. The same effect is also known for lithium intercalated carbon anode secondary batteries (D. Aurbach, Y. Ein-Eli, O. Chusid, Y. Carmeli, M. Babai and H. Yamin, J. Electrochem. Soc. 1994, 141, 603-611). Sulfur dioxide (SO.sub.2) has also been reported to be another additive that improves charge-discharge cycling in rechargeable lithium ion cells (Y. Ein-Eli, S. R. Thomas and V. R. Koch, J. Electrochem. Soc. 1996, 143, L195-L197).
In spite of the success of CO.sub.2 and SO.sub.2 in improving cell discharge characteristics, their use has been limited. One problem associated with both CO.sub.2 and SO.sub.2 as electrolyte additives is that they are in a gaseous state at room temperature, and are thus difficult to handle. Also, it is difficult to control the dissolved concentration of CO.sub.2. Best results are achieved at pressures of up to 50 psig., which further detracts from the practicality of this additive.
Instead of carbon dioxide and sulfur dioxide, the present invention is directed to the provision of organic phosphate additives in the electrolyte of an alkali metal electrochemical cell to beneficially modify the anode surface film. The phosphate additives are defined herein as organic phosphate mono-ester, diester or triester compounds or phosphoric acid provided as a co-solvent with commonly used organic aprotic solvents. The organic phosphate additives are in a condensed phase which makes them easy to handle in electrolyte preparation. When used as a co-solvent in an activating electrolyte, the phosphate additives interact with the alkali metal anode to form an tonically conductive surface protective layer thereon. The conductive surface layer improves the discharge performance of the alkali metal electrochemical cell and minimizes or even eliminates voltage delay in the high current pulse discharge of such cells.